fix a typo.

Rewritten testing introduction, added environment setup to README.md, removed empty pages that were not referenced

changed bootstrap namespace to jui

- Moved everything test-related into `tests` directory. Codeception tests are in `codeception`.
- Removed database module since we're using fixtures and migrations.
- Moved console entry points and bootstrap into `tests/codeception/bin`.
- Adjusted travis build scripts.
- Adjusted documentation to be consistent and reflect changes made.

changed problem to problems to correct a minor typo.

Fixes #4659: adjusted nginx config in docs to serve 404 instead of "no input file specified" when accessing non-existing PHP file

Backslash after alias causes 'Invalid path alias' exception in `\yii\BaseYii::getAlias`

Added support for arbitrary operators to Query::filterWhere, added support for arbitrary operators to Sphinx extension